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Topic: Volume knob question. (Read 999 times) previous topic - next topic

Sloth6

Hello,
I am currently working on a project that uses live text-speech conversion with the speakjet chip. It is curently wired as following.
http://letsmakerobots.com/files/SpeakJetTTS_Schematic.PNG

Note that this is from droidbuilders project on lests make robots.

Anyway, I have a 10k pot for volume control, but when I turn it I get a horrible screeching noise instead of a lowered volume. What could be causing this and are there standard fixes?

As a side question - and perhaps related to my problem. This setup is for an 8 ohm speaker. Would it make any difference to have 4 speakers, 2 pairs in parallels which are in series instead of one? The equiv resistance is the same so I assumed it is fine. Bad call?

Thanks a lot.

Osgeld

#1
May 27, 2012, 09:27 am Last Edit: May 27, 2012, 09:29 am by Osgeld Reason: 1
screeching from a LM386????

no seriously this is a finicky little PITA amp chip and if the cosmos does not align right that is a sheer fire way of letting you know. I have never personally bothered figuring it out, though I am sure others have but I have worked around it in a couple different ways, totally ghetto of course.

one was just to fiddle with a trimpot then hard wire the input divider so it would shut up, and then change the amplitude of the input.

the other oddly enough was to put a LED in parallel with the power going to the chip.

personally I hate the thing, but thats just me, and there is a real way to solve the problem, maybe someone can chime in.

Quote
Bad call?


depends on what you want
http://arduino.cc/forum/index.php?action=unread;boards=2,3,4,5,67,6,7,8,9,10,11,66,12,13,15,14,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,86,87,89,1;ALL

Grumpy_Mike

You need to put a capacitor across the power pins of the chip, perhaps a 100uF or larger.

Sloth6

Thank you both for your replies. I tried a 100uf and 200uf and sadly the squeaking remains. At this point I may just remove the 10k pot.

Grumpy_Mike

I would not give up untill you have reached 1000uF

Sloth6

Also it turns out my potentiometer was fried, didn't even know that was possible at 5v but it no longer works. Sigh, thanks for your help. I will try it now with a working pot and a bunch of caps.

General electronics question while I'm at it. In this design some caps are placed in series on the line (like the 100 and 10uf) while others are used as a voltage divider with ground. Clearly the voltage divider ones will have a larger voltage across them. Is the distinction to filter out different frequencies? My cap knowledge is lacking.

thanks :)

Grumpy_Mike

You can fry a pot easly if you wire it wrong.
A cap is just a frequency dependant resistor, in that circuit it is used to act as a low pass filter and also as a DC blocker for AC coupling.

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